Silicon Gratings Research Articles

High coupling efficiency polarization splitting waveguide grating couplers.

We propose and validate a new, to the best of our knowledge, approach for high coupling efficiency polarization splitting waveguide grating couplers (PSWGCs). The high coupling efficiency is achieved by using a polysilicon overlay grating, which is shifted with respect to the underlying silicon grating. The PSWGC is designed to couple the two orthogonal polarizations in different directions. The design simulations predicted the coupling efficiency to be -1.46 dB for transverse electric (TE) polarization and -1.88 dB for transverse magnetic (TM) polarization with a single grating coupler, while the experimentally measured coupling efficiency was -2.37 dB and -2.51 dB for TE and TM polarizations. The high coupling efficiency GC is achieved without using bottom metal reflectors, and the design of the PSWGC is fully compatible with large-volume manufacturing using photolithography typically available in commercial silicon photonic foundries.

Tunable and polarization-dependent electromagnetically induced transparency analogy in graphene-based terahertz metasurfaces

In this paper, we theoretically and numerically demonstrate a tunable and polarization-dependent electromagnetically induced transparency analogy based on graphene terahertz metasurfaces. The unit cell of the metasurface consists of three-layer graphene strips embedded in a silicon grating. The dynamic adjustment of the transparent window can be achieved by changing the coupling distance between the graphene layers and the polarization direction of the incident lights. The operation mechanism behind the phenomenon can be attributed to the near-field interaction and electromagnetic coupling of modes in graphene strips. Furthermore, the full wave electromagnetic simulations obtained by the finite-difference time-domain method agree well with the theoretical fitting results based on the three-harmonic oscillator model. In addition, by changing the Fermi levels, it can not only realize the outstanding slow-light effects with a maximum group index of 3750 but also obtain the four-frequency asynchronous optical switch function in terahertz regions. Therefore, our proposed metamaterial device may have potential applications in image switching, optical switches, slow-light device, optical communication, and optical storage.

Seed-guided high-repetition-rate femtosecond laser oxidation for functional three-dimensional silicon structure fabrication

Laser oxidation provides a flexible maskless functional three-dimensional silicon structure fabrication strategy. Here, oxide layers are fabricated on silicon substrates by high-repetition-rate femtosecond laser irradiation. Formation mechanisms of the femtosecond laser oxidation are analyzed to tune the oxide layer morphology. When the laser fluence is high, the oxidation process is accompanied by laser ablation, so the oxide layer is always on top of laser ablated groove. To obtain the flat oxide layer, defect is firstly generated on silicon substrate by the laser irradiation at a high laser fluence of 126 mJ/cm2, function as a high optical absorption. It is called seed as it can assist to achieve lower fluence laser oxidation at 38 mJ/cm2 and result in a flat oxide layer. Full width at half-maximum of the oxide layer is also reduced. The laser fabricated oxide layers are used as etching masks. Three-dimensional silicon gratings are fabricated by deep reactive ion etching, which demonstrate desirable beam diffraction properties.

Tunable nonreciprocal metasurfaces based on a nonlinear quasi-bound state in the continuum.

Nonreciprocal devices are essential and crucial in optics for source protection and signal separation. A hybrid grating system consisting of a silicon grating, a graphene layer, and a silicon waveguide layer is employed to create a high-Q quasi-BIC (bound state in the continuum). Then, the high-Q properties of the quasi-BIC are harnessed to enhance the third-order nonlinear effect of silicon, thereby improving the nonreciprocal characteristics of the device. The nonreciprocal transmittance ratio of the device can be tunable by adjusting the graphene Fermi energy level, achieving tunability ranging from 0.0865 to 30.57 dB. It also enables the best performance of the device over a wider range of frequency bands. This study provides a new, to the best of our knowledge, method for designing tunable nonreciprocal devices with a wide range of potential applications.

Sidewall Corrugation-Modulated Phase-Apodized Silicon Grating Filter.

In this work, phase-apodized silicon grating filters with varying sidewall corrugation width and location were investigated, while the resonance wavelength, extinction ratio, and rejection bandwidth were tuned flexibly. The grating filters with a waveguide width of 500 nm and grating period of 400 nm were fabricated and characterized as a proof of concept. The resonance wavelength of the device can be shifted by 4.54 nm by varying the sidewall corrugation width from 150 to 250 nm. The corresponding rejection bandwidth can be changed from 1.19 to 2.03 nm by applying a sidewall corrugation location offset from 50 to 200 nm. The experimental performances coincide well with the simulation results. The presented sidewall corrugation-modulated apodized grating can be expected to have great application prospects for optical communications and semiconductor lasers.

Fully Connected Feedforward Neural Network for the Prediction of Amorphous Silicon Grating Couplers Efficiency

Photonic circuits are an enabling technology for the development of novel solutions in different fields such as healthcare, quantum computing, neural networks, communications, and manufacturing. Interconnections between devices and systems require low-loss light coupling strategies. Grating couplers are a promising solution to couple light between photonic circuits and optical fibres due to their off-plane coupling capabilities. Hydrogenated amorphous silicon (a-Si:H), which can be deposited by PECVD over a substrate of silica or glass, is a suitable low-cost solution for the production of such light coupling devices. In this work we developed, trained and tested a fully connected feedforward neural network for coupling efficiency prediction in a-Si:H grating couplers. The light coupling gratings were simulated by twodimensional finite-difference time-domain (FDTD) analysis and field distributions were analysed with the Finite Element Method (FEM). Simulated gratings include non-apodized, linear and quadratic refractive index variation designs featuring full or partial etching, operating at 1550 nm. Not featuring any type of bottom reflector, the couplers exhibit coupling efficiencies up to about 40 % (~ -4 dB). The neural network multiclass grating coupler efficiency classifier was trained with over 3000 simulation results, reaching an accuracy over 85%, for coupling efficiencies between 0 and 30%+.

Blazed Silicon Gratings for Soft X-Ray and Extreme Ultraviolet Radiation: the Effect of Groove Profile Shape and Random Roughness on the Diffraction Efficiency

Blazed Silicon Gratings for Soft X-Ray and Extreme Ultraviolet Radiation: the Effect of Groove Profile Shape and Random Roughness on the Diffraction Efficiency

Quasi-bound states in continuum in double-layer silicon gratings

Bound states in the continuum (BICs) are theoretically known to possess infinite lifetimes and Q factor. However, due to the difficulties in achieving it in reality, symmetry breaking is often introduced in the structure to transform symmetrically protected BICs into quasi-BICs (q-BICs) with extremely high Q factor. Therefore, q-BICs can be utilized to enhance the Q factor of optical sensors. In this paper, we propose the design of a double-layer composite one-dimensional grating with a high Q factor. The structure consists of a double-layers silicon (Si) grating on a silicon dioxide (SiO2) substrate. By introducing a displacement in the upper-layer grating to break the symmetry, q-BICs are induced. The induced q-BICs achieve a Q factor of 2248 for transverse magnetic (TM) wave, enabling enhanced optical sensing capabilities. The proposed q-BICs sensor, exhibiting anisotropy for both TM and transverse electric wave (TE), holds great potential for narrowband polarizers and sensing applications.

All-dielectric metasurface refractive index sensor with high figure of merit based on quasi-bound states in the continuum

All-Dielectric metasurfaces are widely used in the field of sense due to their small size, freedom of design, and low radiation loss. However, the optical sensor still has the problem of wide full width at half maximum (FWHM), which leads to a low figure of merit (FOM). In order to solve this problem, an all-dielectric metasurface based on quasi-bound states in the continuum (q-BIC) was proposed for refractive index sensing. The sensor structure consisted of a prism/metasurface/analyte, where the metasurface was composed of periodically arranged subwavelength asymmetric silicon gratings. The asymmetry of the structure led to the coupling of the tangential and radiative field components in the superficial layer at the resonance position, which formed a q-BIC resonance peak with a narrow FWHM in the reflection spectrum. We explained the causes of the q-BIC resonance peak generation by finite element analysis, and analyzed the factors affecting the FWHM, and also simulated the machining errors. The results showed that by reducing the asymmetry of the sensor, it could have a high FOM, with a FOM of 1.35 × 107 RIU−1. In addition, simulated machining errors within ±5 nm showed that small machining errors did not lead to the disappearance of q-BIC, which proved that the sensor had good robustness. These findings had provided ideas for the accurate detection of trace substances.

C- and O-Band Dual-Polarization Fiber-to-Chip Grating Couplers for Silicon Nitride Photonics.

Highly efficient coupling of light from an optical fiber to silicon nitride (SiN) photonic integrated circuits (PICs) is experimentally demonstrated with simple and fabrication-tolerant grating couplers (GC). Fully etched amorphous silicon gratings are formed on top of foundry-produced SiN PICs in a back-end-of-the-line (BEOL) process, which is compatible with 248 nm deep UV lithography. Metallic back reflectors are introduced to enhance the coupling efficiency (CE) from -1.11 to -0.44 dB in simulation and from -2.2 to -1.4 dB in experiments for the TE polarization in the C-band. Furthermore, these gratings can be optimized to couple both TE and TM polarizations with a CE below -3 dB and polarization-dependent losses under 1 dB over a wavelength range of 40 nm in the O-band. This elegant approach offers a simple solution for the realization of compact and, at the same time, highly efficient coupling schemes in SiN PICs.

Embedded silicon gratings for high-efficiency light-chip coupling to thin film silicon nitride waveguides.

Thin film silicon nitride (<150 nm) waveguide has emerged as a dominant ultra-low-loss platform for many loss-critical applications. While thin-film silicon nitride propagation loss is a crucial characteristic, coupling light between an optical fiber and the waveguide is still challenging. While the larger mode size of the decoupled thin waveguide offers better coupling than a highly-confined waveguide, the coupling efficiency is still sub-optimal. The poor diffraction efficiency of such thin films limits the scope of implementing standalone surface gratings. We demonstrate an efficient way to couple into thin film silicon nitride waveguides using amorphous silicon strip gratings. The high contrast gratings provide an efficient means to boost the directionality from thin films leading to an enhanced coupling performance. In addition, we incorporate a bottom reflector to further improve the coupling. We present an optimal design for uniform strip gratings with a maximum coupling efficiency of -1.7 dB/coupler. We achieved a maximum coupling efficiency of -0.28 dB/coupler by engineering the scattering strength along the grating through apodization. We have experimentally shown the highest coupling efficiency reported yet of -2.22 dB/coupler and -1.84 dB/coupler for uniform and apodized grating couplers in the C-L band. We present a detailed design strategy, simulation, fabrication and characterization data on the effect of various parameters on the coupling efficiency.

Monolithic Quantum Dot Discrete Mode Laser on SOI

Single-longitudinal-mode light sources on a silicon platform exhibit tremendous potential in data transmission, integrated on-chip optical interconnects, and optical ranging. A unique InAs/GaAs quantum dot discrete mode (DM) laser directly grown on a silicon-on-insulator (SOI) substrate with embedded silicon gratings has been demonstrated here, which is regrowth-free and compatible with integrated silicon photonics. Attributed to refractive index perturbation and optical feedback from embedded silicon gratings prepatterned on the SOI substrate, this DM laser can provide stable single-mode operation with a threshold current of 45 mA and an output power of 9 mW. The maximum optical signal-to-noise ratio (OSNR) achieved is 43.6 dB with an optical linewidth of 157 kHz. The 3 dB direct modulation bandwidth is measured as 3.6 GHz. The DM laser can be externally modulated at 70 Gbit/s with non-return-to-zero (NRZ) modulation format and 80 Gbit/s under 4-level pulsed amplitude (PAM4) modulation format. Such SOI-based DM laser paves the way toward large-scale and high-power on-chip integrated lasers.

Smith–Purcell Radiation from Highly Mobile Carriers in 2D Quantum Materials

AbstractTerahertz (THz) radiation has broad applications ranging from medical imaging to spectroscopy. One viable source of high‐intensity THz radiation is the Smith–Purcell (SP) effect, which involves charge carriers moving over a periodic surface. Conventional SP emitters use electron beams to generate charge carriers, necessitating bulky electron acceleration stages. Here, a compact design for generating THz SP radiation using mobile charge carriers within 2D materials is proposed. This circumvents the beam alignment and beam divergence challenge, allowing for a reduction in the electron‐grating separation from tens of nm to 5 nm or less, leading to more efficient near‐field excitation and a potentially chip‐level THz source. In such a configuration, it is shown that the optimal electron velocity and the corresponding maximum radiation intensity can be predicted from the electron‐grating separation. The numerical demonstration shows that hot electrons can excite SP radiation in graphene on a silicon grating, and the radiation intensity can be increased by graphene surface plasmons. This study can be extended to a broad variety of charge carriers in 2D materials, thus allowing for compact, tunable, and low‐cost THz sources.

Mid-infrared circular-polarization-sensitive photodetector based on a chiral metasurface with a photothermoelectric effect.

Photothermoelectric conversion in chiral metasurfaces with thermoelectric material provides an effective way to achieve circular polarization recognition. In this paper, we propose a circular-polarization-sensitive photodetector in a mid-infrared region, which is mainly composed of an asymmetric silicon grating, a film of gold (Au), and the thermoelectric B i 2 T e 3 layer. The asymmetric silicon grating with the Au layer achieves high circular dichroism absorption due to a lack of mirror symmetry, which results in a different temperature increasing on the surface of the B i 2 T e 3 layer under right-handed circularly polarized (RCP) and left-handed circularly polarized (LCP) excitation. Then the chiral Seebeck voltage and output power density are obtained, thanks to the thermoelectric effect of B i 2 T e 3. All the works are based on the finite element method, and the simulation results are conducted by the Wave Optics module of COMSOL, which is coupled with the Heat Transfer module and Thermoelectric module of COMSOL. When the incident flux is 1.0W/c m 2, the output power density under RCP (LCP) light reaches 0.96m W/c m 2 (0.01m W/c m 2) at a resonant wavelength, which achieves a high capability of detecting circular polarization. Besides, the proposed structure shows a faster response time than that of other plasmonic photodetectors. Our design provides a novel, to the best of our knowledge, method for chiral imaging, chiral molecular detection, and so on.

Fano-Like Resonance of Heat-Reconfigurable Silicon Grating Metasurface Tuned by Laser-Induced Graphene.

We propose a heat-reconfigurable metasurface composed of the silicon-based gold grating. The asymmetric Fano-like line shape is formed due to the mutual coupling of the local surface plasmon (LSP) in the gap between the two layers of Au gratings and the surface propagating plasmon (SPP) on the surface of the Au gratings. Then, we effectively regulate the Fano resonance by applying a bias voltage to laser-induced graphene (LIG), to generate Joule heat, so that the resonant dip of one mode of the Fano resonance can shift up to 28.5 nm. In contrast, the resonant dip of the other mode barely changes. This effectively regulates the coupling between two resonant modes in Fano resonance. Our study presents a simple and efficient method for regulating Fano-like interference in the near-infrared band.

Combined polysilicon and silicon gratings for dual-wavelength-band waveguide grating couplers.

We proposed a novel, to the best of our knowledge, design for a dual-wavelength-band waveguide grating coupler. The proposed structure works in both the C band and O band. The proposed device is optimized from an initial design of two independent gratings formed on the silicon and polysilicon overlay layers, respectively. We designed the up layer (polysilicon) for the C band and the down layer (silicon) for the O band as the initial optimization seed. After numerical optimization of this structure using a genetic algorithm, the grating coupler has a coupling efficiency of -3.86 dB at the C band and -4.46 dB at the O band. We validate the approach in a commercial foundry using 193-nm photolithography in a multi-project wafer, and the experimental result has coupling efficiencies of -4.37 dB in the C band and -5.8 dB in the O band.

Physics-based Models for photonic thermometers.

Resistance thermometry, meticulously developed over the last century, provides a time-tested method for taking temperature measurements. However, fundamental limits to resistance-based approaches along with a desire to reduce the cost of sensor ownership, increase sensor stability and meet the growing needs of emerging economy has produced considerable interest in developing photonic temperature sensors. In this study we utilize Della-Corte-Varshni treatment for thermo-optic coefficient to derive models for temperature-wavelength relationships in silicon ring resonators and Fiber Bragg gratings. Model evaluation is carried out using a Bayesian criteria that selects models for superior out-of-sample predictive accuracy whilst minimizing model complexity. Our work presents physics-based framework for photonic thermometry reference functions, putting constraints on model complexity and parameter bounds, pointing the way towards a reference function that can be utilized for future standardization and inter-comparison of photonic thermometers.

Free-electron terahertz radiation based on silicon gratings with in-plane waveguide emission

We present a method utilizing the coupling between a pre-bunched electron beam and a silicon subwavelength grating to generate coherent terahertz waves. The grating that is connected to two opposite-traveling in-plane waveguides functions as a resonator. An example operating around 2 THz shows that, when the velocity and repetition frequency of the electron bunches respectively match the phase velocity and resonant frequency of the Bragg resonance in the grating, the strong electron-wave coupling leads to coherent radiation through the waveguide. The repetition frequency of the electron bunches can be halved by using its second harmonic to match the resonant frequency. This study might offer a potential approach for on-chip terahertz sources.

A High-Performance Tactical-Grade Monolithic Horizontal Dual-Axis MEMS Gyroscope With Off-Plane Coupling Suppression Silicon Gratings

This articledemonstrates the developments toward a high-performance monolithic wheeled structure horizontal dual-axis [pitch ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X-</i> axis) and roll ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</i> -axis)] microelectromechanical system (MEMS) gyroscope based on off-plane coupling suppression silicon gratings. Gyroscopes have been fabricated with a silicon-on-glass fabrication process with vacuum sealed in metal packaging. The featured gyroscope operates in split mode with the bandwidths of two axes devices both of ∼70 Hz. By unitizing two individual active restraining control loops that adjust the quadrature amplitude via differentially biasing the silicon gratings in real time, the off-plane mechanical couplings of the two sense modes are significantly reduced by ∼95 and ∼30 times, which enables ∼25-fold and ∼56-fold improvements in the bias instability (BI) for a typical dual-axis device. Tactical-grade performances are achieved for both the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X-</i> axis and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y-</i> axis gyroscopes with angle random walk of 0.02 deg/√h and 0.045 deg/√h along with in-run BI of 0.26 deg/h and 0.65 deg/h, respectively, without any temperature compensation. The current preliminary results show great potential to achieve navigation grade precision by employing the mode-matching technique in the future. The use of silicon gratings for out-of-plane coupling reduction also provides an effective way to build a high-performance planar monolithic chip inertial measurement unit.

Dual Tunable Electromagnetically Induced Transparency Based on a Grating-Assisted Double-Layer Graphene Hybrid Structure at Terahertz Frequencies.

We propose a graphene plasmonic structure by applying two graphene layers mingled with a thin gold layer in a silicon grating. By utilizing the finite-difference time-domain (FDTD) method, we investigate the optical response of the system, and observe that the design achieves dual tunable electromagnetically induced transparency (EIT)-like effect at terahertz frequencies. The EIT-like effect arises from the destructive interference between the grapheme-layer bright modes and the gold-layer dark mode. The EIT-like phenomenon can be adjusted by the Fermi level, which is related to the applied voltage. The results show that the group delay of the present structure reaches 0.62 ps in the terahertz band, the group refractive index exceeds 1200, the maximum delay-bandwidth product is 0.972, and the EIT-like peak frequency transmittance is up to 0.89. This indicates that the device has good slow light performance. The proposed structure might enable promising applications in slow-light devices.